Process for producing corrosion resistant coating of barium ferrate on ferrous metals



United States Patent PROCESS FOR PRODUCING CORROSION RESIST- ANT COATINGOF BARIUM FERRATE ON FER- ROUS METALS Paul H. Margulies, Princeton, andWilliam J. Tillis, Levittown, N.J., assignors to FMC Corporation, acorporation of Delaware No Drawing. Filed Apr. 24, 1961, Ser. No.104,780

7 Claims. (Cl. 20456) This invention relates to the production of aresistant coating on ferrous metals, and more particularly to theapplication of a ferrate coating on the surfaces of ferrous metals.

Many methods are available for preventing corrosion of ferrous metalsurfaces. The most popular commercial method involves placing aconversion coating on the surface of the ferrous metal to insulate itfrom a corrosive atmosphere. These conversion coatings result from thedeposition of metal compounds on the surface of the workpiece.

One of these conversion coatings which is of interest for preventingcorrosion of ferrous metals is the ferrate film. Ferrate films have beenproduced on ferrous metal surfaces in the past by maintaining theferrous metal surface (or other metal surface) in a bath of sodium orpotassium ferrate for about 1 hour. These soluble ferrates, e.g., sodiumferrate, exist in solution in an unstable condition and deposit on theimmersed workpiece, leaving a thin yellow coat of sodium ferrate (Na FeOThis process is disclosed in U.S. Patent No. 2,850,416, issued to JohnE. Castle on September 2, 1958.

While these films afford some measure of corrosion resistance, themethod of application as well as the degree of corrosion protectionafforded by these films has not been commercially acceptable. The use ofunstable solutions requiring freshly made chemicals, and the lengthytime for producing the film is a serious drawback when this process issought to be commercially exploited. Moreover, the sodium ferrate orpotassium ferrate which is deposited on the workpiece, does not givehigh corrosion resistance vis-a-vis other commercial conversioncoatings, e.g., phosphate coatings. Another drawback to this process isthat it permits only soluble ferrate salts to be employed even thoughother ferrates are known, since the ferrate must form on the workpiecefrom a prepared solution of the ferrate. Obviously insoluble ferratescannot be employed, since they cannot be dissolved.

It is an object of the present invention to form a corrosion resistant,insoluble ferrate-coating in-situ on the surface of ferrous metals froma stable liquid solution,

in a short time.

These and other objects will be apparent from the following description.

It has now been determined, quite unexpectedly, that corrosion resistantfilms of barium ferrate (BaFeO can serves as the anode, and a conductorserves as the cathode.

The one-formation of an insoluble barium ferrate insitu on the surfaceof the anodic workpiece is quite un- 3,20%,384 Patented Sept. 14, 1965expected, since the prior process required all the reagents, includingthe iron which goes into the coating as well as the soluble sodium orpotassium salt, to be dissolved in the treating solution. By contrast,the present process only uses an alkaline solution of a soluble bariumsalt, a barium hydroxide or a barium oxide, the iron in the coatingbeing supplied by the anodic ferrous workpiece. The insoluble bariumferrate compound which results is formed in-situ at the surface of theferrous anode and instantly deposits on this surface as an adherent,corrosion resistant layer. The barium ferrate layer is highly effectiveas a corrosion resistant surface, being more resistant than certaincommercial conversion coatings such as iron phosphates, and offeringcorrrosion resistance at least equal to commercially employed phosphatecoatings, such as zinc phosphate.

In the operation of the present process an alkaline solution of a bariumsalt, a barium hydroxide or a barium oxide, preferably in aconcentration of A5 N or above, constitutes the electrolyte. The maximumallowable concentration of a barium salt, a barium hydroxide or a bariumoxide which may be employed is limited only by the solubility of thecompound in solution. The barium compound which has been found mostuseful is Ba(OH) -8H O, although barium oxide, hydrated barium oxides,and anhydrous barium hydroxide can be employed. Soluble barium saltssuch as barium acetate, barium nitrate, and barium chloride, are alsoeffective when used in alkaline solutions. The barium salt, bariumhydroxide or barium oxide solution will yield barium ferrate per se, byanodic treatment of a ferrous workpiece, as long as the barium solutionis alkaline. However, in order for the production of a corrosionresistant film of the newly formed barium ferrate on the surface ofanodic workpiece, the barium salt, barium hydroxide or barium oxidesolution must have a high alkalinity. In general, the corrosionresistance of the barium ferrate film increases with higher alkalinityuntil an optimum alkalinity is reached. It has been found that uniformfilms of BaFeO, having good corrosion resistance are obtained withbarium salt, barium hydroxide or barium oxide solutions having pHs ofabout 12.5 and above. The films obtained at lower pH values tend to bethinner and to be non-uniform. While these latter films also offer somemeasure of corrosion resistance and can be used in this application,they have not been found to be as efficacious as the films produced frombarium salt, barium hydroxide or barium oxide solutions having pHs ofabout 12.5 and above.

The film-producing electrolyte may be made up using Ba(OH) alone, ormixed with an alkali metal hydroxide. When Ba(OH) is used alone, asolution in the range of /5 N to about 1 N should be employed sincesolutions containing more than 1 N Ba(OH) exceeded the practicalsolubility limits and are wasteful of barium, while concentrations below/5 N result in films which are too thin for completely adequatecorrosion protection. The addition of an alkali metal hydroxide to thebarium solution is the preferred method of operation since more uniformcoatings are obtained with this electrolyte. The alkali metal hydroxide,e.g., NaOH or KOH, is added within the range of 0.5 N to 7.5 N, to a l Nsolution of Ba(OH) When an alkali metal hydroxide is added to theelectrolyte an additional advantage is obtained, over and above that ofobtaining more uniform coatings. This is in the '3 lowering of theminimum concentration of barium salt from A; N to as low as N, requiredfor the successful production of coatings. An electrolyte containingabout N NaOH and about 1 N Ba(OH) has been found most suitable.

The ferrous metal workpiece to be coated is placed within theelectrolyte, and is connected to a source of direct current. The ferrousobject constitutes the anode, while a metallic conductor which isresistant to the electrolyte serves as the cathode e.g. steel sheets,carbon, platinum. If desired a suitable metal container used to hold theelectrolyte solution can constitute the cathode.

A potential of at least about 1.4 volts must be applied before filmformation begins. Flms of barium ferrate will form at all potentialsabove about 1.4 volts. The minimum current which is required to producethese films is about 2 amperes per square foot of anodic surface(a.s.f.). The exact amount of current which can be employed depends uponfactors such as voltage and internal resistance of the cell. Currents ashigh as 360 a.s.f. have been employed successfully.

Barium ferrate fihns which are applied to ferrous metal surfaces at fromabout 2 to about 4 volts exhibit good resistance against surfacecorrosion. The films may be applied at potentials of 2 volts with onlyabout 5 to a.s.f. At potentials of about 4 volts, a.s.f. values of about144 have been found to give good results. Coatings prepared for the samelength of time, at about 2 volts and at about 4 volts, are virtuallyidentical except that those prepared at about 2 volts appear to have amore uniform surface when subject to a microscopic examination.

While the exact reason for the deposition of a more uniform coating atlow electromotive potentials is not known,

it is believed due to the less rapid formation of microbubbles at 2volts then at 4 volts, which results in little or no microsizedirregularities on the surface of the film. Nevertheless, films depositedat the higher potentials offer good corrosion resistance againstcorrosive atmospheres,

while those deposited at lower potentials offer good resistance againstboth corrosive atmospheres and water immersion.

The films of the present process can be formed with the electrolyticbath at temperatures of about 160 to 220 F. At these temperatures thefilm begins depositing as soon as current is applied. The preferred timeand temperatures have been found to be about 1 to 12 minutes and about180 to 190 F. The temperature of the filming electrolyte, and the timeof deposition, are extremely important since the thickness of the filmwhich deposits on the ferrous metal surface is dependent upon both ofthese variables. The thickness of the film increases with highertemperatures and with longer deposition periods.

The films which are produced at 2 volts are not as thick as thoseapplied at 4 volts under identical condition. However, since thethickness of the film is dependent upon both the treating time and thetemperature of the electrolyte, it is possible to obtain the desirablyuniform films produced at about 2 volts without sacrificing filmthickness, either by increasing the temperature of the 10 containing 1 NBa(OH) can uniformly coat, on a conservative basis, over 10,000 squarefeet of ferrous metal surface. With respect to the power requirement, 2.potential of 4 volts applied with 144 a.s.f. of anode surface for 4minutes, requires 0.0385 kilowatt hour for filming 5 each square foot.If a potential of 2 volts at 15 a.s.f. of

anode surface is employed for 4 minutes, only 0.002 kilowatt hours arerequired per square foot of anode surface.

The following examples are given to illustrate the invention, but arenot deemed limitative of it.

EXAMPLE 1 Three SAE-l020 panels, 1 /2 by 1 /2 by .050 inch were cleanedin 20% HCl until all surface oxides were removed. The panels were themrinsed in water and placed in a steel beaker containing a solution of 39g. of

40 g. of NaGH, and enough water to make 1 liter of solution. Each of thepanels was used as the anodic ter minal and the steel beaker was used asthe cathodic terminal of a source of direct current. The temperature ofthe solution was maintained at F., and 144 amperes per square foot wasapplied at a potential of 4 volts. At the termination of the 4 minutes,the panels were covered with a uniform red barium ferrate film. Thesefilmed panels were rinsed, dried, and placed in a closed atmospherecontaining 0.05% sulfur dioxide, alongside identical uncoated panels.The sulfur oxide was produced by decomposing sodium pyrosulfite withacid. The coated and uncoated test specimens were maintained in thisatmosphere at 40 C. for 24 hours. The coated panels showed only theslightest evidence of corrosion, whereas the uncoated panels showedsevere corrosion.

The above procedure was repeated using cast iron and 314 stainless steelpanels. The results obtained were the same as will the SAE-10'20 steelpanels.

EXAMPLE 2 Steel panels, identical to those of Example 1, 1% by 1 /2 by.050 inch were cleaned in 20% HCl until all surface oxides were removed.These panels were removed and placed in steel beakers containing thesolutions set forth in Table I. The panel was used as the anodicterminal, and the steel beaker was used as the cathodic terminal of asource of direct current. The temperature of the solutions, the voltage,and the amperage are given in Table I. The coated panels were subjectedto microscopic examination to determine the extent of any microscopicpores or breaks in the uniform surface of the coating. The results aregiven in Table I.

Table I Solution components Volts a.s.f. Tempera- Time, Microscopicture, F. min. examination Ba(OH)z NaOH 1 2 3. 6 178 4 Few pits. 1 0. 5 N2 3. 6 176 4 Few pits. 1 1. 0 N 2 5. 8 180 4 Few pits. 1 2 N 2 8. 6 1804 Few pits. 1 3 N 2 5. 8 179 4 Very few pits. 1 5 N 2 8. 6 180 4 Veryfew pits. 1 N 7. 5 N 2 10.0 180 4 Very few pits. 1 N 10 N 2 7. 2 178 4Few pits. 1 N 12. 5 N 2 11.0 180 4 Many pits. 1 N 15 N 2 7.2 180 4 Manypits.

5 EXAMPLE 3 The procedure of Example 2 was repeated, using differentpotentials for comparison. The conditions of operation are given inTable H. The coated panels were subenamel. The barium ferrate coatingacts as an intermediate bonding layer, and effectively binds surfacecoatings such as paints, to the base metal more tenaciously than if thepaint were applied directly to the metal surface. In

jected to microscopic examination, and were also tested 5 additionOrganic enamel applied as finish coat for corrosion resistance conductedunder the conditions over. a .pnmer coatlpg of barium ferrate i ferrategiven below The results of these tests are given in cpatmg Ldeleienouslyffiected by the balimg opera- Table H t1on whlch 1s requiredto yield an enamel having a hard,

s02 VAPOR oo n aosron surface- The coated anels were'suspended in aclosed dessica- 1O uisuant to requirements of the patient Statutes thetor containing o 05% sulfur dioxide This quantity of i i mvemw beenexplained and. Sulfur dioxide a reduced b the r'eaction of 025 plrfied1n a manner so that it can be readlly practiced by f Th those skilled inthe art, such exemplification including 0 so mm 9 2 3: g 3 6 E 3 what isconsidered to represent the best embodiment of Yf i d t or theinvention. However, it should be clearly understood ours m 15 Sim a e mus na a mosp that, within the scope of the appended claims, the inven-HUMIDITY *TE'ST tion may be practiced by those skilled in the art, and

Coated panels were suspended in a dessicator kept at havlPg the benefitof this dlscloure othefwlse than as 100% relative humidity at 40 C. Testpanels were inspeclficaliy dfiicnbeci f exemplified herem' spected after48 hours exposure. What claimed p v q 1. The process of coating thesurface of a ferrous WATER i i metal with a corrosion resistant coatingof barium ferrate Coated p f were Immersed 1n udlvldual beakers whichcomprises immersing said ferrous metal in an alkacontaining distilledwater at a pH of 7.0:02. So1ut1o'ns line solution containing from about0.5 N to about 7.5 N were kept at ambient temperature for 48 hours. ofan alkali metal hydroxide and barium ions in amounts Table II SolutionCompounds Corrosion Tests Volts a.s.i. Microsoeopic S02 H20 H20examination Ba (0H); NaOH Vapor Vapor Immer- 4 140 Good Good PoorNumerous pores. 1 N- 4 130 Good Good Poor Numerous pores. 2 15 E" E* 15*Very few pores. 1 N 2 15 E* E* E* Very few pores.

Excellent.

While the barium ferrate coatings as obtained in the preceding examplesare found to be excellent corrosion resistant films, it has beendetermined that after-treatments are capable of improving the corrosionresistance of the film. In particular, an after-treatment dip of theferrous metal coated with the BaFeO film in a dilute dichromatesolution, materially increases the corrosion resistance of the coating.The dichromate, and preferably sodium dichromate, can be employed inaqueous solutions as low as about 0.2% by weight. Higher amounts can beemployed if desired with the same beneficial effect. 7

EXAMPLE 4 Several panels of SAE-1020 steel were coated with a BaFeO filmby anodic deposition. The films were formed by applying a potential of 2volts at 10.5 (a.s.f.) for a period of 4 minutes. The electrolytecontained 1 N Ba(OH) plus 5 N Na(OH) and was maintained at 95 C. Onehalf of these coated panels were then subjected to an after-treatment byimmersion for seconds in a 0.2% by weight aqueous solution of sodiumdichromate maintained at 50 C. The solution was neutralized to a pH of7.0 just prior to the after-treatment. At the conclusion of theafter-treatment, both the treated and untreated samples were immersed inindividual beakers of distilled water at a pH of 7.0i0.2, and at ambienttemperatures. The time required for the appearance of a inch diametercorrosion spot was considered as the time of failure. The untreatedsamples required 50 hours to fail, Whereas the after-treated samplesrequired 200 hours to fail.

The barium ferrate coatings, in addition to being good corrosionresistant films, also have been found suitable as base coatings forfinish coatings of paint, lacquer or of from about N up to thesolubility limit of barium ions in said alkaline solution, said alkalinesolution having a pH of at least about 12.5, passing an electric currenthaving a potential of,at least 1.4 volts and in an amount of at last 2amperes per square foot of ferrous metal sur face through said ferrousmetal, said alkaline solution serving as the electrolyte, said ferrousmetal serving as the anode, and a conductor which is resistant to theelectrolyte serving as the cathode.

2. The process of coating the surface of a ferrous metal with acorrosion resistant coating of barium ferrate which comprises immersingsaid ferrous metal in an alkaline solution containing barium ions inamounts of from about /go N to about 1 N and an alkali metal hydroxidein amounts of from about 0.5 N to about 7.5 N, said alkaline solutionhaving a pH of at least about 12.5, passing an electric current having apotential of at least 1.4 volts and in an amount of at least 2 amperesper square foot of ferrous metal surface through said ferrous metal,said alkaline solution serving as the electrolyte, said ferrous metalserving as the anode, and a conductor which is resistant to theelectrolyte serving as the cathode.

3. The process of claim 2 wherein the barium ions are supplied by addingbarium hydroxide.

4. Process of claim 2 wherein said electric current has a potential ofabout 1.4 to 4 volts and is applied at the rate of 5 to 20 amperes persquare foot of ferrous metal surface.

5. Process of claim 2 wherein said solution contains barium hydroxide ina concentration of about 1 N and an alkaline metal hydroxide inconcentrations of about 0.5 N to 7.5 N.

6. The process of claim 2 wherein said ferrous metal coated with saidbarium ferrate is after-treated by being dipped in an aqueous solutionof sodium dichromate con- 7 8 taining at least about 0.2% by weightsodium dichromate. as the anode, and a conductor which is resistant tothe elec- 7. Process of coating the surface of a ferrous metal trolyteserving as the cathode. with a corrosion resistant coating of bariumferrate which comprises immersing said ferrous metal in a solution con-References Cited by the Examiner tainin about 1 N barium h droxide andabout 5 N of an alkalir ie metal hydroxide, saiid solution having a pHof at 5 FOREIGN PATENTS least 12.5, passing an electric current throughsaid ferrous 692,124 6/40 Germany.

metal, said current having a potential of about 1.4 to about 2 volts andbeing applied at a rate between 5 to 20 am- JOHN MACK, Primary Examiner-'peres per square foot of ferrous metal surface, said solu- 10 JOSEPHREBOLD, Examiner tion serving as an electrolyte, said ferrous metalserving UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No3 ,206,384 September 14, 1965 Paul H. Margulies et al.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below Column 1, line 65, for "one-formation" read one-stepformation column 2, line 55, for "exceeded" read exceed column 3, line14, for "Flms" read Films line 36, for "then" read than column 6, line45, for "last" read least Signed and sealed this 9th day of August 1966(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Offioer Commissioner ofPatents

1. THE PROCESS OF COATING THE SURFACE OF A FERROUS METAL WITH ACORROSION RESISTANT COATING OF BARIUM FERRATE WHICH COMPRISES IMMERSINGSAID FERROUS METAL IN AN ALKALINE SOLUTION CONTAINING FROM ABOUT 0.5 NTO ABOUT 7.5 N OF AN ALKALI METAL HYDROXIDE AND BARIUM IONS IN AMOUNTSOF FROM ABOUT 1/20N UP TO THE SOLUBILITY LIMIT OF BARIUM IONS IN SAIDALKALINE SOLUTION, SAID ALKALINE SOLUTIONHAVING A PH OF AT LEAST ABOUT12.5, PASSING AN ELECTRIC CURRENT HAVING A POTENTIAL OF AT LEAST 1.4VOLTS AND IN AN AMOUNT OF AT LAST 2 AMPERES PER SQUARE FOOT OF FERROUSMETAL SURFACE THROUGH SAID FERROUS METAL, SAID ALKALINE SOLUTION SERVINGAS THE ELECTROLYTE, SAID FERROUS METAL SERVING AS THE ANODE, AND ACONDUCTOR WHICH IS RESISTANT TO THE ELECTROLYTE SERVING AS THE CATHODE.